Thermoplastic-toughened cyanate ester resin composites with low heat release properties

ABSTRACT

Composite materials that contain thermoplastic-toughened cyanate ester resins as the resin matrix. The composite materials exhibit low levels of heat release when burned. The matrix resins are composed of from 50 to 80 weight percent of a cyanate ester resin component. The matrix resin composition also includes from 10 to 40 weight percent of a thermoplastic blend that is composed of polyetherimide and polyamideimide. The epoxy resin composition further includes from 1 to 10 weight percent of a curative agent. The composite materials may be used for primary structures in aircraft and other load-bearing structures.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to composite materials thatcontain a thermoplastic-toughened cyanate ester resin as the resinmatrix. These high-strength composites are suitable for use as primarystructures in aircraft and other load-bearing applications. The presentinvention is directed to the properties of such composite materials whenthey are burned. In particular the invention involves providing suchthermoplastic-toughened cyanate ester composites that have low heatrelease properties and short self-extinguishing times.

2. Description of Related Art

Cyanate ester resins that are reinforced with a fibrous material, suchas glass or carbon fiber, are used in a wide variety of situations wherehigh structural strength and low-weight are required. Compositematerials that use a high performance cyanate ester resin matrix areespecially popular in the aerospace industry where weight and structuralstrength are important engineering and design considerations. Highperformance cyanate ester resins typically include one or morethermoplastic materials that provide “toughening” of the cyanate esterresin. Although such high performance cyanate ester resin compositematerials are desirable because of their relatively high strength toweight ratio, they do present some specific issues with respect to heatrelease, flammability and other burn properties.

A major goal in developing formulations for high performance cyanateester resin composites is to limit the amount of heat that is releasedduring burning, while at the same time not reducing the structuralstrength of the cured composite part. This is especially important forprimary structures and parts that are located in the interior areas ofaircraft. It is also important that any attempt to reduce heat releasedoes not adversely affect properties of the uncured cyanate ester resin,such as tack and viscosity. The tack and viscosity of the uncured resinare especially important when the cyanate ester resin is used to makeprepreg, which is a common intermediate material used in the fabricationof aerospace parts.

Heat release requirements for composite materials, which are used in theinterior of aircraft, are set forth in the Federal AviationAdministration (FAA) Aircraft Materials Fire Test Handbook (14 C.F.R25.853(d), Appendix F, Part 1) and BOEING Specification Support StandardBSS-7322. The total rate of heat release during combustion of a testsample is measured as well as the peak heat release rate during theburning process. The standard test used to determine the heat releaserequirements for composite materials exposed to radiant heat is the OhioState University (OSU) heat release test. It is desirable to providecyanate ester resin composites that have an average OSU total heatrelease rate at two minutes of less than 65 kilowatt-minutes per squaremeter (kw-min/m²) and a peak OSU heat release rate that is less than 65kw-min/m². These two OSU values are the minimum requirements set by 14C.F.R. 25.853(d), Appendix F Part IV, for interior aircraft parts madefrom composite materials.

Resistance to surface flammability is also an important area of concernfor high performance cyanate ester resin composites. It is importantthat a cyanate ester resin composite part, which is on fire, be able toself-extinguish once the source of heat and/or flame is removed. Theability to self-extinguish is an especially important consideration forprimary structures and parts located in the interior areas of aircraft.It is also a significant goal of cyanate ester resin formulators todevelop cyanate ester resins that are used to make composites whichself-extinguish in as short a time period as possible, while at the sametime keeping structural strength of the finished composite part at thelevels needed for aerospace applications. The same requirement that thetack and viscosity of the uncured cyanate ester resin not be adverselyaffected applies with respect to attempts to formulate cyanate esterresins with short self-extinguishing times.

The United States Federal Aviation Administration has establishedregulations and requirements for fire resistance of aircraft interiorparts and materials. These requirements are set forth in 14 C.F.R.25.853(a). One requirement is that the composite material be able toself-extinguish once the flame source is removed. The test procedure formeasuring the self-extinguishing time for cyanate ester resins are alsoset forth in the FAA Aircraft Materials Fire Test Handbook (FAR 25.853,Appendix F, Part 1) and in BOEING Specification Support StandardBSS-7230 (Revision H), which is recognized in the aerospace industry asa standard test method. It would be desirable to provide highperformance cyanate ester resin composites where the self-extinguishingtimes are as short as possible and at least below 15 seconds. A 15second self-extinguishing time is the maximum allowed for compositeaircraft parts pursuant to 14 C.F.R. 25.853(a).

SUMMARY OF THE INVENTION

In accordance with the present invention, it was discovered that cyanateester resin compositions, which have a particular blend of thermoplastictoughening agents, can be combined with a fibrous support and cured toprovide composite materials that have low OSU heat release rates andshort self-extinguishing times when compared to existing highperformance toughened cyanate ester resin composites.

The compositions of the present invention are composed of a fibrousreinforcement and a resin matrix that contains from 50 to 80 weightpercent of a cyanate ester resin component that includes one or morecyanate ester resins. The matrix resin also includes from 10 to 40weight percent of a thermoplastic blend comprising polyetherimide andpolyamideimide wherein the weight ratio of polyetherimide topolyamideimide is from 5:1 to 1:1. A curative agent in an amount rangingfrom 0 to 10 weight percent is also included.

The present invention covers prepreg and other compositions that includethe uncured or partially cured resin matrix and a fibrous reinforcement.In addition, the invention covers cured composite parts where the resinmatrix has been cured. The composite parts are well-suited for use asprimary structure in aircraft and other load-bearing applications wherehigh strength is required. Parts and structures made using compositematerials in accordance with the present invention are particularlywell-suited for use as primary structures that are located in theinterior of aircraft.

The invention also covers methods for making compositions that containuncured matrix resin and fibrous support as well as the cured parts andproducts that incorporate the matrix resin composition.

The above described and many other features and attendant advantages ofthe present invention will become better understood by reference to thefollowing detailed description when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an aircraft, which depicts exemplaryexterior primary aircraft structures that can be made using compositematerials in accordance with the present invention.

FIG. 2 is a partial view of a helicopter rotor blade, which depictsexemplary exterior primary aircraft structures that can be made usingcomposite materials in accordance with the present invention.

FIG. 3 is a simplified view of an exemplary T-stiffener structure thatcan be made using composite materials in accordance with the presentinvention. The T-stiffener is a primary structure that is used in theinterior of aircraft.

FIG. 4 is a simplified view of an exemplary cargo floor flange structurethat can be made using composite materials in accordance with thepresent invention. The cargo floor flange is a primary structure that isused in the interior of aircraft.

FIG. 5 is a simplified view of an exemplary aircraft clip structure thatcan be made using composite materials in accordance with the presentinvention. The clip is a primary structure that is used in the interiorof aircraft.

FIG. 6 is a simplified view of an exemplary flange support structurethat can be made using composite materials in accordance with thepresent invention. The flange support is a primary structure that isused in the interior of aircraft.

DETAILED DESCRIPTION OF THE INVENTION

Matrix resin compositions in accordance with the present invention maybe used in a wide variety of situations where a thermoplastic-toughenedcyanate ester resin is desired. Although the cyanate ester resincompositions may be used alone, the compositions are generally combinedwith a fibrous support to form composite materials. The compositematerials may be in the form of a prepreg or cured final part. Althoughthe composite materials may be used for any intended purpose, they arepreferably used in aerospace vehicles and particularly preferred for usein commercial and military aircraft. For example, the compositematerials may be used to make non-primary (secondary) interior aircraftstructures, such as aircraft galley and lavatory parts and as windowframes, floor panels, overhead storage bins, wall partitions, wardrobes,ducts, ceiling panels and interior sidewalls. In addition, the compositematerials may be used to make primary aircraft structures. Primaryaircraft structures or parts are those elements of either fixed-wing orrotary wing aircraft that undergo significant stress during flight andwhich are essential for the aircraft to maintain controlled flight. Thecomposite materials may also be used to make “load-bearing” parts andstructures in general.

FIG. 1 depicts a fixed-wing aircraft at 10 that includes a number ofexemplary primary aircraft structures and parts that may be made usingcomposite materials in accordance with the present invention. Theexemplary primary parts or structures include the wing 12, fuselage 14and tail assembly 16. The wing 12 includes a number of exemplary primaryaircraft parts, such as ailerons 18, leading edge 20, wing slats 22,spoilers 24 trailing edge 26 and trailing edge flaps 28. The tailassembly 16 also includes a number of exemplary primary parts, such asrudder 30, fin 32, horizontal stabilizer 34, elevators 36 and tail 38.FIG. 2 depicts the outer end portions of a helicopter rotor blade 40which includes a spar 42 and outer surface 44 as primary aircraftstructures. Other exemplary primary aircraft structures include wingspars, and a variety of flanges, clips and connectors that connectprimary parts together to form primary structures.

The composite materials of the present invention are particularlywell-suited for making primary aircraft structures that are located inthe interior of the aircraft where low OSU heat release values and shortself-extinguishing times are an especially important consideration. Anexemplary T-stiffener 11, which is a primary structure that is locatedin the interior of the aircraft, is shown in FIG. 3. The T-stiffener 11is made up of horizontally oriented layers 13 of composite material thatform the flange portion of the stiffener and L-shaped layers ofcomposite material 15 and 17 that form the rib portion of theT-stiffener. The T-stiffener 11 may include holes (not shown) forattaching the T-stiffener to the flange and rib surfaces using bolts orrivets to provide stiffening of the flange-rib assembly.

Another exemplary interior primary aircraft structure, which can be madeusing composite materials in accordance with the present invention, is acargo floor flange support, as shown at 21 in FIG. 4. The cargo flangesupport 21 is used to connect the cargo floor represented in phantom at23 to the aircraft fuselage represented in phantom at 25. The cargoflange support 21 includes a first flange portion 27 and second flangeportion 29, both of which include holes (31 and 33, respectively) forattachment to the aircraft. The cargo floor flange support 21 alsoincludes reinforcing ribs 35 and 37, which provide needed stiffness andstructural strength to the part. An exemplary aircraft clip structure isshown at 70 in FIG. 5. The clip structure 70 is designed to connect twoprimary interior structures together. The two primary aircraftstructures are shown in phantom at 72 and 74. Holes 76 are provided forconnecting the clip 70 to the primary structures using bolts or rivets.

Another exemplary interior primary aircraft structure, which can be madeusing composite materials in accordance with the present invention, is aflange support structure 41 that is shown in FIG. 6. The flange supportstructure 41 is designed to connect two primary aircraft parts together.The two aircraft parts 43 and 45 are shown in phantom in FIG. 6. Theaircraft parts 43 and 45 are connected to the flange support 41 by wayof bolting or riveting through holes 46 or by adhesive bonding

The resin composition that is used to form the resin matrix includesfrom 50 to 80 weight percent of a cyanate ester resin componentcomprising one or more cyanate ester resins. Preferably, the resinmatrix will include from 55 to 65 weight percent of the cyanate esterresin component. The cyanate ester resins that make up the cyanate esterresin component may be any of the cyanate ester resins that are amenableto thermoplastic toughening. Such cyanate ester resins are well-known inthe aerospace industry. Exemplary cyanate ester resins includebisphenol-E cyanate ester resin, bisphenol-A cyanate ester resin,hexafluorobisphenol-A cyanate ester resin, tetramethylbisphenol-Fcyanate ester resin, bisphenol-C cyanate ester resin, bisphenol-Mcyanate ester resin, phenol novolac cyanate ester resin anddicyclopentadienyl-bisphenol cyanate ester resin. Bisphenol-E cyanateester resins are preferred. The relative amounts and types of cyanateester resins used in a particular resin composition may be varied.However, in a preferred embodiment, bisphenol-E cyanate ester resin isused as the sole cyanate ester resin in the cyanate ester resincomponent.

Cyanate ester resins are available commercially from a number ofsources. For example bisphenol-E cyanate ester resin is available fromHuntsman under the trade name AroCy L-10. Bisphenol-A cyanate esterresin, hexafluorobisphenol-A cyanate ester resin andtetramethylbisphenol-F cyanate ester resin are also available fromHuntsman under the trade names AroCy B-10, AroCy F-10 and AroCy M-10,respectively. Bisphenol-C cyanate ester resin, bisphenol-M cyanate esterresin, phenol novolac cyanate ester resin anddicyclopentadienyl-bisphenol cyanate ester resin are available fromHuntsman under trade names AroCy RD98-228, AroCy XU-366, AroCy XU-371and XU-71787.02L.

The matrix resin composition also includes from 10 to 40 weight percentof a thermoplastic blend comprising polyetherimide (PEI) andpolyamideimide (PAI). It is preferred that the matrix resin compositioncontain from 20 to 30 weight percent of the thermoplastic blend. Thethermoplastic blend in accordance with the present invention includesone thermoplastic compound (i.e. PEI) that is soluble in the cyanateester resin component and the other (i.e. PAI) that is not soluble inthe cyanate ester resin component. The relative amounts of PEI and PAImay be varied between weight ratios (PEI: PAI) of 5:1 to 1:1.Preferably, the weight ratios (PEI: PAI) will vary between 4:1 and 2:1.It was found that the total and relative amounts of PEI and PAI withinthese ranges provides an effective way to reduce both the maximum (peak)and total heat release rates for composites containing thermoplastictoughened cyanate ester resins as the resin matrix.

Polyetherimide is available commercially as ULTEM 1000P from Sabic(Dubai). Polyamideimide is available commercially as TORLON 4000T orTORLON 4000TF from Solvay Advanced Polymers (Alpharetta, Ga.). PEIthermoplastics are typically supplied as powders where the PEI particlesrange in size from about 30 to 300 microns. The particle size of the PEIparticles is not particularly important, since the PEI powder isdissolved in the cyanate ester component during preparation of the resincomposition. Commercially available PAI powders typically have averageparticle sizes 50 μm.

The matrix resin composition is prepared by mixing the PEI particles andPAI particles with the cyanate ester resin component and heating theresulting mixture to a temperature of around 95° C. for a sufficienttime to completely dissolve the PEI particles. The PAI particles do notdissolve. The time necessary for the PEI particles to dissolve will varyfrom 10 minutes to one hour or more depending upon the size of the PEIparticles and the dissolution temperature. If desired, the PEI particlesmay be dissolved first and then the PAI particles are added.

After the PEI particles are dissolved, the mixture is cooled to 90° C.or below and the curative agent component (and PAI particles, if notpreviously added) is added to form a resin composition that is ready forcombination with a fibrous support and curing. The particle size andamount of PAI is selected so that the viscosity of the cyanate esterresin composition is within a range that is suitable for prepregpreparation. The preferred average particle size range for the PAIparticles is from 8 microns to 20 microns. Accordingly, it is preferredthat commercially available PAI powder be ground or otherwise processedin order to provide a powder having the desired smaller particle size.It is preferred that the viscosity of the resin be the same as theviscosity of existing high performance toughened resins that arepresently used in the aerospace industry to make prepreg includingquasi-isotropic chopped prepreg.

The amount of curative agent component that is added to the cooledmixture of cyanate ester resin component and thermoplastic blend is from0 to 10 weight percent of the total weight of the cyanate ester resincomposition and preferably from 2 to 5 weight percent. Any of thecurative agents that are used to provide curing ofthermoplastic-toughened cyanate ester resin may be used to cure theresin composition. Exemplary curative agents in accordance with thepresent invention include bisphenol sulfone and bisphenol A. The resinmay be cured without the using a curative agent, if desired.

The cyanate ester resin composition may also include additionalingredients, such as performance enhancing and/or modifying agentsprovided that they also do not adversely affect the viscosity and tackof the uncured resin so as to make it unsuitable for making prepreg. Thecyanate ester resin composition also contains from 1 to 15 weightpercent of a compatible fire retardant agent. The fire retardant agentis a cyanate ester resin that includes phosphorous in the back bone ofthe cyanate ester resin. Such phosphorous-containing cyanate ester resinfire retardant agents are well-known in the art. The amount of fireretardant that is added may be varied. However, it is preferred that theamount of fire retardant that is added be at least sufficient to insurethat the maximum and total OSU heat release rates are below at least 65kw-min/m². Preferred amounts of such phosphorous-containing cyanateester resin are in the range of 8 to 12 weight percent of the totalresin composition.

Phosphorous-containing cyanate ester resin fire retardants are availablecommercially from a number of sources. It is preferred that the cyanateester resin contain from 5 to 15 weight percent phosphorous in thepolymer backbone. An exemplary fire retardant is Primaset FR-300, whichis available from LONZA (Basel, Switzerland). FR-300 is a crystallinesolid that has a molecular weight of 374 and a melting point of 135° C.FR-300 has a phosphorous content of about 9 weight percent and a geltime at 200° C. of over 20 minutes. The glass transition temperature(T_(g)) is over 300° C. and the exotherm (DSC) is 206° C. FR 300 is apreferred phosphorous-containing cyanate ester resin fire retardant.

The cyanate ester resin compositions of the present invention are madein accordance with standard resin processing procedures for highperformance toughened cyanate ester resins. The cyanate ester resin orresins are mixed together at room temperature or at an elevatedtemperature to melt crystals. The PEI and PAI portions of thethermoplastic component are then added. This mixture is then heated aspreviously described to a temperature at which the PEI is dissolved. Themixture is then cooled down to 90° C. or below and the curative agent(if any), the phosphorous-containing cyanate ester resin fire retardantand other additives (if any), are mixed into the resin to form the finalresin composition that is impregnated into the fiber reinforcement toform the resin matrix.

The cyanate ester resin composition is applied to the fibrousreinforcement in accordance with any of the known prepreg manufacturingtechniques. The fibrous reinforcement may be fully impregnated with thecyanate ester resin composition. The prepreg is typically covered onboth sides with a protective film and rolled up for storage and shipmentat temperatures that are typically kept well below room temperature toavoid premature curing. Any of the other prepreg manufacturing processesand storage/shipping systems may be used, if desired.

The fibrous reinforcement of the prepreg may be selected from hybrid ormixed fiber systems that comprise synthetic or natural fibers, or acombination thereof. The fibrous reinforcement may be any suitablematerial such as fiberglass, carbon or aramid (aromatic polyamide)fibers. The fibrous reinforcement is preferably carbon fibers.

The fibrous reinforcement may comprise cracked (i.e. stretch-broken) orselectively discontinuous fibers, or continuous fibers. The fibrousreinforcement may be in a woven, non-crimped, non-woven, unidirectional,or multi-axial textile structure form, such as quasi-isotropic choppedpieces of unidirectional fibers. The woven form may be selected from aplain, satin, or twill weave style. The non-crimped and multi-axialforms may have a number of plies and fiber orientations. Such styles andforms are well known in the composite reinforcement field, and arecommercially available from a number of companies, including HexcelReinforcements (Villeurbanne, France). For example, plain weave carbonfiber fabrics identified as AGP193-P and SGP193-P are suitable fibrousreinforcements that are commercially available from HexcelReinforcements.

The prepreg may be in the form of continuous tapes, towpregs, webs, orchopped lengths (chopping and slitting operations may be carried out atany point after impregnation). The prepreg may be an adhesive orsurfacing film and may additionally have embedded carriers in variousforms both woven, knitted, and non-woven. The prepreg may be fully oronly partially impregnated, for example, to facilitate air removalduring curing. The amount of resin matrix (resin content) in the prepregmay vary from 20 to 60 weight percent of the total prepreg weight. Resincontents on the order of 30 to 45 weight percent are preferred.

The prepreg may be molded using any of the standard techniques used toform composite parts. Typically, one or more layers of prepreg are placein a suitable mold and cured to form the final composite part. Theprepreg of the invention may be fully or partially cured using anysuitable temperature, pressure, and time conditions known in the art.Typically, the prepreg will be cured in an autoclave at temperatures ofbetween 160° C. and 190° C. with curing temperatures of between about175° C. and 185° C. being preferred. Curing times and temperatures maybe varied depending upon the amount (if any) and type of curative agentthat is present in the resin composition. Compression molding ofquasi-isotropic chopped prepreg or molding material that contain thethermoplastic-toughened cyanate ester resin is a preferred procedure.The quasi-isotropic chopped prepreg is the same as HexMC® compressionmolding material that is available from Hexcel Corporation (Dublin,Calif.), except that the resin component of this quasi-isotropic choppedprepreg is made in accordance with the present invention. Suchquasi-isotropic materials are described in EP 113431 B1 and U.S. patentapplication Ser. No. 11/476,965. Unidirectional prepreg, alone or incombination with woven prepreg or quasi-isotropic chopped unidirectionalprepreg is also a preferred material for making load bearing parts.Unidirectional fiber tape is available from a variety of commercialsources. For example AS4GP unidirectional fiber tape is available fromHexcel Corporation (Dublin, Calif.).

It is preferred that the composite part be post-cured at a temperatureof 190° C. to 220° C. for at least one hour. It was found thatpost-curing of composite materials in accordance with the presentinvention provides additional lowering of the heat release rates ascompared to composite materials that are not post-cured. Post-curing ispreferably conducted in a convection oven. However, any suitablepost-curing procedure may be used. It is preferred that the compositematerial be post-cured at about 200° C. for about 2 hours.

Cyanate ester resin compositions of the present invention will have theviscosity and tack that is required in order for the resin to be used inthe formation of prepreg. The viscosity of the resin compositions shouldbe similar to existing high performance toughened epoxy resins, such asHexPly® resins 8552. The tack should be sufficiently low to allow theprepreg to be handled for transport and storage, while at the same timebeing sufficiently high to allow multiple layers of prepreg to beapplied and used with molds in accordance with known prepregmolding/curing procedure. When used as the resin matrix for a moldingcompound, such as quasi-isotropic chopped prepreg, the resin should haveviscosity and tack properties that are similar to existing matrixresins, such as HexPly® resins 8552.

Preferred resin compositions have the following formulation:

-   -   55 to 65 weight percent bisphenol-E cyanate ester resin;    -   20 to 30 weight percent thermoplastic blend of PEI:PAI (4:1 to        2:1);    -   8 to 12 weight percent phosphorous-containing cyanate ester        resin fire retardant agent    -   0 to 5 weight percent bisphenol sulfone

These preferred resin compositions, when used as the resin matrix forcomposite material, provide maximum and total OSU heat release ratesthat are well below the 65 kw-min/m² heat release rate maximums set by14 C.F.R. 25.853(d) and self-extinguishing time limit of 15 seconds setby 14 C.F.R. 25.853(a), while still retaining desired prepreg handlingproperties in accordance with the present invention. When thesepreferred resins are used to make composite parts that form primaryaircraft parts or structures, it is preferred that the fibrousreinforcement is made from carbon fibers and that the prepreg issubjected to post-curing.

Particularly preferred epoxy resin compositions are those that have thefollowing formulation: 1) a cyanate ester resin component made up offrom 58 to 62 weight percent of AroCy L-10 (bisphenol-E cyanate esterresin); 2) a thermoplastic component made up of from 19 to 23 weightpercent polyetherimide and from 4 to 8 weight percent polyamideimide; 3)from 8 to 12 weight percent FR 300 or similar phosphorous-containingcyanate ester resin; and 4) from 2 to 4 weight percent bisphenolsulfone. These particularly preferred resin compositions, when used asthe resin matrix for composite material, provide peak and total OSU heatrelease rates that are well below the 65 kw-min/m² heat release ratemaximums set by 14 C.F.R. 25.853(d) and the self-extinguishing timelimit of 15 seconds set by 14 C.F.R. 25.853(a), while still retainingdesired prepreg handling properties in accordance with the presentinvention. When these particularly preferred resins are used to makecomposite parts that form primary aircraft parts or structures, it ispreferred that the fibrous reinforcement is made from carbon fibers andthat the prepreg is subjected to post-curing.

If desired, small amounts of other thermoplastic may be used incombination with PEI or PAI. For example, in situations where theaircraft primary structure is not located in the interior of theaircraft, polyethersulfone (PES) and other sulfur containingthermoplastic materials may be added to supplement and/or replace aportion of the PEI in the thermoplastic blend. Amounts of PES or othersulfur containing thermoplastics should not form more that 25 weightpercent of the “PEI” portion of the thermoplastic blend. Polyamide (PA)and other similar thermoplastics may be added to supplement and/orreplace a portion of the PAI in the thermoplastic blend. Amounts of PAor other similar thermoplastics should not form more than 25 weightpercent of the “PAI” portion of the thermoplastic blend.

Examples of practice are as follows:

EXAMPLE 1

A resin composition having the following formulation was prepared foruse in forming the resin matrix of an exemplary composite material:

-   -   60.4 weight percent bisphenol-E cyanate ester resin (AroCy L-10)    -   10.0 weight percent phosphorous-containing cyanate ester resin        (FR300)    -   20.7 weight percent polyetherimide (ULTEM 1000P)    -   6.0 weight percent polyamideimide (TORLON 4000TF)    -   2.9 weight percent bisphenol sulfone (BPS)

The liquid cyanate ester resin AroCy L-10 and PEI particles were mixedtogether at room temperature and heated to 110° C. for 60 minutes inorder to completely dissolve the PEI particles. The mixture was cooledto 100° C. and PAI particles were added. The mixture was further cooledto 85° C. and FR-300 and BPS were added. The weight ratio ofpolyetherimide to polyamide for this example is 3.45:1. For all of theexamples, TORLON TF powder was ground and sieved to provide PAI powderhaving an average particle size of 15 μm.

The matrix resin was used to form a test sample by impregnating 12layers of AS4C 193AW plain weave carbon fiber fabric with the matrixresin films to provide a pre-preg that was 0.25 cm thick. The pre-pregcontained 38 percent by weight resin matrix. The pre-preg was cured inan autoclave at 177° C. for 120 minutes to form a cured compositematerial. The cured composite material was cut into a test sample thatwas 150 cm×150 cm×0.25 cm. In addition, some of the cured compositematerial was post-cured at 200° C. for two hours in a convection oven.The post-cured composite material was cut into a post-cured test samplethat was also 150 cm×150 cm×0.25 cm. Both the cured and post-curedsamples were tested for OSU heat release rate in accordance with 14 CFR25.853d, Appendix F, Part IV. The cured test sample had a peak OSU heatrelease rate of 64 kw-min/m² and a total OSU heat release rate of 23kw-min/m². The post-cured sample had a maximum OSU heat release rate of54 kw-min/m² and a total OSU heat release rate of 26 kw-min/m². As canbe seen from this example, post-curing provides a substantial drop (−10kw-min/m²) in the peak OSU heat release rate while causing only a slightincrease (+3 kw-min/m²) in the total OSU heat release rate.

Both the cured and post-cured composite test samples were subjected to a60 second vertical burn test to determine the self-extinguishing time inaccordance with the modified Method F of BSS-7230 (Revision H). Both thecured and post-cured test samples had a self-extinguish time of 4seconds.

EXAMPLE 2

Pre-preg was made in the same manner as Example 1 except that nophosphorous-containing cyanate ester resin (FR300) was included and theamount of bisphenol-E cyanate ester resin (AroCy L-10) was increased to70.4 weight percent to make up for the missing FR300. The pre-preg wascured and post-cured in the same manner as Example 1 and test samplesprepared in accordance with Example 1. The cured test sample had a peakOSU heat release rate of 77 kw-min/m² and a total OSU heat release rateof 64 kw-min/m². The post-cured sample had a peak OSU heat release rateof 69 kw-min/m² and a total OSU heat release rate of 20 kw-min/m². Inaccordance with the present invention, it is preferred that thecomposite be post-cured. As can be seen from this example, post-curingprovides a significant drop in both maximum and total heat release ratesas compared to the test sample that was not post-cured. In addition, oneshould include a sufficient amount of phosphorous-containing cyanateester resin fire retardant in order to reach peak and total OSU heatrelease rates that are below 65 kw-min/m². For this exemplary compositematerial, FR300 should be added in an amount that would reduce the OSUpeak and total heat release rates to below 65 kw-min/m² regardless ofwhether post-curing was used.

Both the cured and post-cured composite test samples were subjected to a60 second vertical burn test to determine the self-extinguishing time inthe same manner as Example 1. The cured sample had a self-extinguishtime of 7 seconds and the post-cured test sample had a self-extinguishtime of 6.5 seconds.

EXAMPLE 3

A resin composition having the following formulation was prepared foruse in forming the resin matrix of an exemplary composite material:

-   -   67.1 weight percent bisphenol-E cyanate ester resin (AroCy L-10)    -   11.1 weight percent phosphorous-containing cyanate ester resin        (FR300)    -   11.9 weight percent polyetherimide (ULTEM 1000P)    -   6.7 weight percent polyamideimide (TORLON 4000TF)    -   3.2 weight percent bisphenol sulfone (BPS)

Pre-preg was made in the same manner as Example 1. The weight ratio ofpolyetherimide to polyamide for this example is 1.78:1. The pre-preg wascured and post-cured in the same manner as Example 1. A test sample ofthe post-cured material was prepared in accordance with Example 1. Thepost-cured sample had a peak OSU heat release rate of 62 kw-min/m² and atotal OSU heat release rate of 32 kw-min/m².

The post-cured composite test sample was subjected to a 60 secondvertical burn test to determine the self-extinguishing time in the samemanner as Example 1. The post-cured test sample had a self-extinguishtime of 4.7 seconds.

EXAMPLE 4

A resin composition having the following formulation was prepared foruse in forming the resin matrix of an exemplary composite material:

-   -   60.4 weight percent bisphenol-E cyanate ester resin (AroCy L-10)    -   12.9 weight percent phosphorous-containing cyanate ester resin        (FR300)    -   20.7 weight percent polyetherimide (ULTEM 1000P)    -   6.0 weight percent polyamideimide (TORLON 4000TF)    -   0.0 weight percent bisphenol sulfone (BPS)

Pre-preg was made in the same manner as Example 1. The exemplarypre-preg was cured and post-cured in the same manner as Example 1. Atest sample of the post-cured material was prepared in accordance withExample 1. The exemplary post-cured sample had a peak OSU heat releaserate of 51 kw-min/m² and a total OSU heat release rate of 18 kw-min/m².

The post-cured composite test sample was subjected to a 60 secondvertical burn test to determine the self-extinguishing time in the samemanner as Example 1. The post-cured test sample had a self-extinguishtime of 2.5 seconds.

COMPARATIVE EXAMPLE 1

A resin composition having the following formulation was prepared foruse in forming the resin matrix of a comparative composite material:

-   -   50.0 weight percent bisphenol-E cyanate ester resin (AroCy L-10)    -   0.00 weight percent phosphorous-containing cyanate ester resin        (FR300)    -   41.1 weight percent polyetherimide (ULTEM 1000P)    -   6.0 weight percent polyamideimide (TORLON 4000TF)    -   2.9 weight percent bisphenol sulfone (BPS)

Pre-preg was made in the same manner as Example 1. The weight ratio ofpolyetherimide to polyamide for this comparative example is 6.85:1,which is outside of the weight ratio range that was found to provideespecially low OSU heat release rates as demonstrated by Examples 1-3.The comparative pre-preg was cured and post-cured in the same manner asExample 1 and test samples prepared in accordance with Example 1. Thecured comparative test sample had a peak OSU heat release rate of 79kw-min/m² and a total OSU heat release rate of 43 kw-min/m². Thepost-cured sample had a peak OSU heat release rate of 82 kw-min/m² and atotal OSU heat release rate of 41 kw-min/m².

Both the cured and post-cured comparative composite samples weresubjected to a 60 second vertical burn test to determine theself-extinguishing time in the same manner as Example 1. The curedsample had a self-extinguish time of 3.0 seconds and the post-cured testsample had a self-extinguish time of 6.0 seconds.

Post curing of the comparative sample did not result in any improvementin OSU heat release rates whereas in Example 2, post-curing producedsubstantial drops in both the peak and total OSU heat release rates.

COMPARATIVE EXAMPLE 2

A resin composition having the following formulation was prepared foruse in forming the resin matrix of a comparative composite material:

-   -   60.4 weight percent bisphenol-E cyanate ester resin (AroCy L-10)    -   10.0 weight percent phosphorous-containing cyanate ester resin        (FR300)    -   26.7 weight percent polyetherimide (ULTEM 1000P)    -   0.0 weight percent polyamideimide (TORLON 4000TF)    -   2.9 weight percent bisphenol sulfone (BPS)

Pre-preg was made in the same manner as Example 1. The comparativepre-preg was cured and post-cured in the same manner as Example 1. Acomparative test sample of the post-cured material was prepared inaccordance with Example 1. The comparative post-cured sample had a peakOSU heat release rate of 70 kw-min/m² and a total OSU heat release rateof 43 kw-min/m².

The post-cured composite test sample was subjected to a 60 secondvertical burn test to determine the self-extinguishing time in the samemanner as Example 1. The post-cured test sample had a self-extinguishtime of 4.7 seconds. When compared to Example 1, this comparativeexample shows that the deletion of polyamideimide from the matrix resincauses a substantial increase in the maximum and total OSU heat releasevalues.

Having thus described exemplary embodiments of the present invention, itshould be noted by those skilled in the art that the within disclosuresare exemplary only and that various other alternatives, adaptations andmodifications may be made within the scope of the present invention.Accordingly, the present invention is not limited by the above-describedembodiments, but is only limited by the following claims.

1. An uncured composite part comprising: a resin matrix comprising: 50to 80 weight percent of a cyanate ester resin component based on thetotal weight of said resin matrix, said cyanate ester resin componentcomprising one or more cyanate ester resins that do not containphosphorous; 10 to 40 weight percent of a thermoplastic blend based onthe total weight of said resin matrix, said thermoplastic blendcomprising polyetherimide and polyamideimide wherein the weight ratio ofpolyetherimide to polyamideimide is from 5:1 to 1:1; 1 to 15 weightpercent of a fire retardant agent based on the total weight of saidresin matrix, said fire retardant agent comprising aphosphorous-containing cyanate ester resin; 1 to 10 weight percent of acurative agent component based on the total weight of said resin matrix,said curative agent component comprising a curative agent for saidcyanate ester component; and a fibrous reinforcement.
 2. An uncuredcomposite part according to claim 1 wherein the resin matrix comprises:55 to 65 weight percent of said cyanate ester resin component based onthe total weight of said resin matrix; 20 to 30 weight percent of saidthermoplastic blend based on the total weight of said resin matrix; 8 to12 weight percent of said fire retardant agent based on the total weightof said resin matrix; and 1 to 5 weight percent of said curative agentcomponent based on the total weight of said resin matrix.
 3. An uncuredcomposite part according to claim 2 wherein the weight ratio ofpolyetherimide to polyamideimide is from 4:1 to 2:1.
 4. An uncuredcomposite part according to claim 1 wherein said curative agent for saidcyanate ester component is selected from the group consisting ofbisphenol sulfone and bisphenol A.
 5. A cured composite part comprisingan uncured composite part according to claim 1 wherein said resin matrixhas been cured.
 6. A cured composite part according to claim 5 whereinsaid cured composite part forms at least part of a primary structure ofan aircraft.
 7. An uncured composite part according to claim 1 whereinsaid fibrous reinforcement comprises unidirectional fibers.
 8. Anuncured composite part according to claim 1 wherein said cyanate esterresin that does not contain phosphorous is selected from the group ofcyanate ester resins consisting of bisphenol-E cyanate ester resin,bisphenol-A cyanate ester resin, hexafluorobisphenol-A cyanate esterresin, tetramethylbisphenol-F cyanate ester resin, bisphenol-C cyanateester resin, bisphenol-M cyanate ester resin, phenol novolac cyanateester resin and dicyclopentadienyl-bisphenol cyanate ester resin.
 9. Anuncured composite part according to claim 1 wherein saidphosphorous-containing cyanate ester resin comprises from 5 to 15 weightpercent phosphorous based on the total weight of thephosphorous-containing cyanate ester resin.